Shape and sources of variations of the functional response of wildfowl: anexperiment with mallards, Anas platyrhynchos

Understanding the variations of the functional response of an organism, i.e . the predation rate in relation to prey density, is necessary to understan d the interactions between the animal and its food supply. This has receive d little attention in dabbling ducks so we investigated experimentally the shape of the functional response of mallard feeding on poultry pellets, and assessed the influence of several factors such as the size of food items, sex or individual performance on this functional response. Individual diffe rences in intake rate are of crucial important in group or gregarious forag ing species. We used two approaches of the functional response: 1) the relation between feeding rate (pellets/s) and pellet densities (pellts/m(2)), and 2) the rel ationship between instantaneous intake rate (g/s) and biomass density (g/m( 2)). For both approaches, we found that the Type II functional response gav e better estimates than a Type I linear functional response but explained o nly a third of the variance. Our results show that pellet size has a large effect on instantaneous intak e rate. The comparison of the functional response parameters suggest that h andling time per prey mag. not reflect the real constraints on intake rate, but that handling time per gram ingested may be more appropriate to integr ate the effect of item size in the functional response. We then discuss the possible mechanisms involved. We also found individual variations in the F unctional response for each of the experiment, with some consistency in the hierarchy regarding feeding efficiency. We did not find any differences be tween males and females. Our results provide an evaluation of individual va riations in intake rate in interference-free conditions, which has rarely b een done, and call for more controlled experiments to allow a finer underst anding of the mechanisms of food acquisition in dabbling ducks.